The present invention pertains in general to methods and apparatus for proximity effect heat-tracing and, in particular, to methods and apparatus for detecting abnormalities in proximity effect heat-tracing circuits.
Viscous fluids, particularly heavy oil and molten sulfur, are often most conveniently transported by pipelines. Movement of viscous fluids through such pipelines is facilitated by maintaining the fluids at an elevated temperature. Steam- or electrically-heated tracer pipes are commonly welded to pipelines in order to maintain the pipelines at temperatures above the ambient temperature.
One type of electrical heating used in long distance pipelines is proximity effect heating. Proximity effect heating is a form of impedance heating in which an insulated conductor is passed through the interior of a ferromagnetic pipe called a heat tube. An alternating current power source is connected between a first end of the heat tube and a first end of the conductor while a second end of the heat tube is connected to a second end of the conductor. Current flows from the power source through the conductor to the second end of the conductor and returns through the heat tube. Heat is generated by the I.sup.2 R loss of the return current flow, by hysteresis and eddy currents induced within the heat tube wall by the alternating magnetic field around the insulated conductor, and by the I.sup.2 R loss in the insulated conductor.
By a process of electromagnetic induction, the current in the insulated conductor causes the return current to concentrate at the inner surface of the heat tube which is the surface of the tube nearest to the conductor. This phenomenon of current concentration is properly referred to as the proximity effect, although the term "skin effect" is often applied.
The current density declines sharply from the inner to outer surface of the heat tube, so that no significant current flows at the outer surface.
In order to detect abnormal operation, a proximity effect circuit is divided in two parts, according to U.S. Pat. No. 3,571,561. A current transformer is inserted into the circuit for each of the parts. Both of the current transformers are connected to apparatus which detects a current corresponding to a difference between the current in the two parts. Deviations from a normal difference between the currents in the two parts signal abnormalities, which may include breakdown of the insulation of the insulated conductor or an abnormal elevation in temperature in one of the parts.
Because of the expense of the high current, high voltage equipment employed, it is desirable to minimize the amount of detection apparatus required.